The Principle of Least Time

(a) Light could take many different paths from A to B.

In sections 1.5 and 4.4, we saw how in the ray model of light, both refraction and reflection can be described in an elegant and beautiful way by a single principle, the principle of least time. We can now justify the principle of least time based on the wave model of light. Consider an example involving reflection, (a). Starting at point A, Huygens' principle for waves tells us that we can think of the wave as spreading out in all directions. Suppose we imagine all the possible ways that a ray could travel from A to B. We show this by drawing 25 possible paths, of which the central one is the shortest. Since the principle of least time connects the wave model to the ray model, we should expect to get the most accurate results when the wavelength is much shorter than the distances involved - for the sake of this numerical example, let's say that a wavelength is 1/10 of the shortest reflected path from A to B. The table, (b), shows the distances traveled by the 25 rays.

(b) The distance traveled along the 25 possible paths ranges from 10.00 wavelengths to 12.28.

Note how similar are the distances traveled by the group of 7 rays, indicated with a bracket, that come closest to obeying the principle of least time. If we think of each one as a wave, then all 7 are again nearly in phase at point B. However, the rays that are farther from satisfying the principle of least time show more rapidly changing distances; on reuniting at point B, their phases are a random jumble, and they will very nearly cancel each other out. Thus, almost none of the wave energy delivered to point B goes by these longer paths. Physically we find, for instance, that a wave pulse emitted at A is observed at B after a time interval corresponding very nearly to the shortest possible path, and the pulse is not very "smeared out" when it gets there. The shorter the wavelength compared to the dimensions of the figure, the more accurate these approximate statements become.

Instead of drawing a finite number of rays, such 25, what happens if we think of the angle, θ, of emission of the ray as a continuously varying variable? Minimizing the distance L requires

Because L is changing slowly in the vicinity of the angle that satisfies the principle of least time, all the rays that come out close to this angle have very nearly the same L, and remain very nearly in phase when they reach B. This is the basic reason why the discrete table, (b), turned out to have a group of rays that all traveleed nearly the same distance.

As discussed in section 1.5, the principle of least time is really a principle of least or greatest time. This makes perfect sense, since dL/dθ=0 can in general describe either a minimum or a maximum

The principle of least time is very general. It does not apply just to refraction and reflection - it can even be used to prove that light rays travel in a straight line through empty space, without taking detours! This general approach to wave motion was used by Richard Feynman, one of the pioneers who in the 1950's reconciled quantum mechanics with relativity (book 6). A very readable explanation is given in a book Feynman wrote for laypeople, QED: The Strange Theory of Light and Matter.